This is when it's good to have a lighter weight bow for practice and training that is built in the same or similar style to your heavier bow(s). I start out with 40 a day on my 48# bow, usually on 4-6 days per week.
Once I'm feeling ready, I will take my two 58-pounders out for some limited stumping, but I hold off on longer sessions until I'm feeling well tuned in the back muscles.
I know I'm doing it right when I can feel the effect on the back muscles immediately after a practice session, but there's no pain anywhere from my fingertips back through my arm and shoulder to my back.